The issue in this case is probably that you are using ECW. See below Generally first ensure that the mosaic datasets works. It is not clear to me in what you wrote whether that is the case. Create the mosaic dataset of your datasources, ensure all works as required and do any required QC. For large areas with NoData I would recommend that you modify the footprint around the datasets to exclude the non wanted areas This works better then setting 0 as Nodata especially for imagery that is lossy compressed. If setting 0 to NoData you may end up with no data areas in say shadows, where lossy compression may set some other values to 0. Using No data will also result in artifacts at the edges as data near the edge may not be 0 exactly. Once the Mosaic dataset works you can then publish it. Ensure though that that the server has access to the imagery.
The issue with serving as an image service in this case will be ECW files. ECW is a proprietary format with a number of restrictions on use of the libraries used to read the format. The licensing by Erdas does not allow a server product to serve ECW images without an additional license from Erdas. To server ECW files you need to either get special license from Erdas else convert the imagery into other formats for example TIF (BigTIF with YCBCR JPEG compression) will give you images that are a about 50% bigger than the ECW files for the same quality. Performance of TIF with JPEG compression is very good. There is a �??Compress Imagery�?� tool in the Imagery Samples gallery of the resource center (http://resources.arcgis.com/gallery/file/image-management) that you may find very useful.